I have been using keep right! to check consistency of my local map, and a regular warning I encounter is "multiple nodes on the same spot". For example, nodes #3042490495, #3061858823, and #3042490472 are in the same place. Is it possible to merge these using the iD editor?

For Potlatch2 or JOSM you could use J or M (keyboard shortcuts), respectively (see also section in DE:Keep Right Users Guide (German)). The merge function in iD (C) seems not to be able to merge nodes which means that you would need to do that manually (copy over tags, way/relation membership and delete the other node(s)).

I suggest to use a more advanced editor (preferably JOSM) to do fixes based on keepright warnings (you seem to be ready to go beyond iD if you care for keepright).